This invention relates to cutting elements and bits incorporating the same and more specifically to cutting elements having a non-uniform interface between their substrate and cutting layer of ultra hard material.
Typically, cutting elements are cylindrical in shape. They have a cemented tungsten carbide substrate having an upper surface. On the upper face is sintered an ultra hard material such as diamond or cubic boron nitride forming a polycrystalline ultra hard material cutting layer.
Common problems that plague cutting elements and specifically cutting elements having an ultra hard material layer bonded on a cemented carbide substrate, are chipping, spalling, partial fracturing, cracking or delamination of the ultra hard material layer. These problems result in the early failure of the cutting layer (i.e., the ultra hard material layer) and thus, in a shorter operating life for the cutting element. Typically, these problems may be the result of peak (high magnitude) stresses generated on the ultra hard material cutting layer at the region in which the ultra hard material layer makes contact with the earth formations during drilling.
One way to attempt to overcome these problems is to increase the thickness of the ultra hard material. Theoretically, an increase in the ultra hard material layer results in increased cutting element impact and wear resistance. However, an increase in the thickness of the ultra hard material layer may result in delamination of the ultra hard material layer from the substrate. Moreover, as the ultra hard material layer thickness increases, the edges and surfaces of the ultra hard material furthest away from the substrate (e.g., the ultra hard material layer upper surface circumferential edge) are starved for cobalt during the sintering process. Consequently, the strength and ductility of these edges are decreased. Thus, the ultra hard material edges subjected to the highest impact loads will be brittle and have lower impact and wear resistance resulting in the early failure of the cutting layer.
Another problem associated with increasing the thickness of the ultra hard material layer is that, as the ultra hard material volume increases, there is an increase in the residual stresses formed on the ultra hard material due to the thermal coefficient mismatch between the ultra hard material layer and the substrate. The cemented carbide substrate has a higher coefficient of thermal expansion than the ultra hard material. During sintering, both the cemented carbide body and ultra hard material layer are heated to elevated temperatures expanding and forming a bond between the ultra hard material layer and the cemented carbide substrate. The heating causes the substrate to expand more than the ultra hard material. As the ultra hard material layer and substrate cool down, the substrate shrinks more than the ultra hard material because of its higher coefficient of thermal expansion. Consequently, thermally induced compressive stresses are formed on the ultra hard material layer and tensile stresses are formed on the substrate. These stresses may reduce the operating life of a cutting element.
Furthermore, an increase in the volume of the ultra hard material also results in the build-up of residual stresses on the ultra hard material layer/substrate interface due to the difference in shrinkage between the ultra hard material and the substrate caused by the consolidation of the ultra hard material and the consolidation of the substrate after sintering. These residual stresses may also reduce the operating life of a cutting element.
Accordingly, there is a need for a cutting element having an ultra hard material table with improved impact and wear resistance, as well as improved cracking, chipping, fracturing, and exfoliating characteristics and thereby an enhanced operating life.